a. I have only so much money.
(Meaning: I have a limited amount of money.)
b. I have only so many books.
(Meaning: I have a limited amount of books.)

I think (a) and (b) correspond to the meaning sentences I have provided.

But couldc. It can only get so good.
d. It can only get so much better.
mean
It can be improved to a limited degree.
?

I think (c) and (d) only mean:
e. The only thing that can happen is that it will get so good/so much better.
As far as I can see 'so' and 'so much' are intensifiers in (c) and (d) and do not express the idea of a limit.

It's really ambiguous, but to my mind, azz, examples C and D can mean there is a limit to the degree of betterment. It's true, the words "so" and "so much" are intensifiers, but when you add "only" to the mix, it will imply a limitation, depending on the context, as gdwdwrkr says.

We should have anticipated Holiday visitors and put a few snacks and beverages out. Welcome, Shelley. It is always nice to see some old names pop in for a quick conversation. At least, it's always been nice so far. Come to think of it, there's a few I could do without hearing from...but you're not one of them

A,B,C and D all suggest a limit (the way I read them)
A) I have a limited amount of money
B) I have a limited amount of books
C) there is a limit to how good it can get
D) there is a limit to how much it will improve

You wrote:c. It can only get so good.
d. It can only get so much better.
"I think (c) and (d) only mean:
e. The only thing that can happen is that it will get so good/so much better.
As far as I can see 'so' and 'so much' are intensifiers in (c) and (d) and do not express the idea of a limit."[/color]

I think "so" and "so much" are negative intensifiers that just serve to reinforce the idea of a limit (to me).
Get rid of the "so" and you can have them mean what you suggest in E.
It can only become good or get better. It won't turn bad or become worse. That's the only direction it will go and there is no limit to how good or how much better it will become.

Azz, substituting "just" for "only" still seems ambiguous to me. If your aim is to express that something will/can only get better (i.e., a fine wine with age, or a skill with practice), then I think there are less cumbersome, and unambiguous ways to say it:

It will only get better and better.
It gets so good.
It just gets so much better!
It will get so, so good.
It can get sooo good.
It can get so much better.
(The list goes on . . . but wait! There's more!)

I can't explain why the insertion of the words "only" or "just" can suddenly change the meaning from one of limitless improvement to one of limited improvement. I can say only that it just does.

I go along with what all the others have said, especially Shelley, to whom a warm welcome back - did you ever get rid of that plague of pigeons?

The reason for the ambiguity has to do with the scope of the word "only" or "just". As is the case with many expressions that are commonly referred to as "adverbial", the positioning is flexible and will affect the scope, but some positions are preferred, even though a different scope is intended. This is one of these cases.

If you change the position of "only" in your sentence c as follows, the meaning is unambiguous:
"It can get only so good."
Here, the "only" clearly scopes to "so good", and the restriction implied by "only" thus restricts the "so good".

In the position you originally posted, the "only" should scope to "get", and the meaning should be "the only thing that can happen is that it will get so good". HOWEVER, the position of such adverbials immediately before the verb is often preferred, irrespective of the intended scope, thus leading to ambiguity. If the speaker intended the "only" to scope to "so good", then that is what is meant, otherwise not. The context would usually resolve the ambiguity.

There are very many freely available academic publications on the Web that look at the scope of adverbials and the associated ambiguities. Most take a very long time to establish what I wrote above in a couple of sentences...

Here is a real-life example of the scoping ambiguity from a shortly-to-be-published translation I am working on:

"The reports say that there was even ice and snow between the wooden blocks and the load."

The context is of a truck that has lost part of its load because of poor securing. The previous sentence makes it clear that the accident happened in the middle of winter and that there was ice and snow everywhere.

As I have formulated the sentence, and without the benefit of the context of the rest of the text (and the associated photos), the word "even" scopes to "ice and snow", i.e. there was contamination between the wooden blocks and the load, even ice and snow.

The position required to make the intended scoping unambiguous is as follows:

"The reports say that there was ice and snow even between the wooden blocks and the load."

This is, of course, grammatically correct and acceptable. So why did I nevertheless opt for the grammatically ambiguous version?

By far the most natural position for "even" is immediately following the verb. Any other position tends to sound somewhat formal or even stilted. The target audience for the monthly column that I translate is primarily truck drivers and loading staff. And the context within which the sentence occurs identifies the scope of "even" quite unambiguously. The two sentences taken together:
"We are in the middle of winter, and there is ice and snow everywhere, even between the different parts of the load. The reports say that there was even ice and snow between the wooden blocks and the load."

This illustrates nicely how nonsensical it is to simply ask "is this sentence ambiguous?". On its own, an utterance may well be ambiguous, but embedded in a context, it is not.